Silane coupling agents have been known to improve the mechanical properties of filled thermosetting and thermoplastic resins since the late 1940's. These low molecular weight compounds are believed to form chemical links between filler particles and polymer molecules, and as such, they must incorporate functional groups capable of reacting with filler and resin alike. Thus, for example, in the modification of unsaturated polymers, or polymerizable monomers, the silane coupling agent typically also contains an unsaturated radical, such as methacrylate, which can graft onto the polymer through a free-radical reaction (e.g., during cure of the resin). The other reactive groups of the silane coupling agent are typically species such as alkoxy or halide groups on silicon, which, when hydrolyzed, can enter into reaction with surface hydroxyl groups of the filler to form a chemical (probably ionic) bond.
Such a silane coupling agent was disclosed by Plueddemann in U.S. Pat. No. 3,258,477, hereby incorporated by reference, and has survived as an industry standard for fiberglass-filled unsaturated polyesters for many years. Plueddemann teaches the use of a silane having the general formula ##STR1## wherein R is H or the methyl radical and X represents the hydrolyzable group referred to above. This silane coupling agent was shown to improve retention of flexural strength of laminates of glass cloth impregnated with an unsaturated polyester resin after exposure to boiling water for two hours. This result was significant in that it is precisely the rapid deterioration of strength, and often electrical properties, after exposure to water that severely limits the utility of polyester composites when coupling agents are not employed.
A similar improvement in resistance to moisture, albeit under relatively mild conditions (water at 50.degree. C./16 hours) was disclosed by Pepe et al. in U.S. Pat. No. 3,671,562 for a composite of glass beads in phenolformaldehyde resin. Pepe et al. teach a silane coupling agent having a terminal urea radical which is connected to silicon through an organic group. The silicon, in turn, is attached to at least one hydrolyzable group and may also be attached to another silicon through an oxygen linkage to form a siloxane therewith. Remaining free valences on the first silicon atom are bonded to organic groups which may include alkenoyloxyalkyl.
One of the major drawbacks of composites based on traditional polyesters has been the relatively poor resistance to moisture of the base resin. This, in turn, is due to the relative ease of hydrolysis of the ester linkage. In this regard, development of so-called "corrosion resistant" polyesters, such as the isophthalates, teraphthalates and acrylic-modified epoxies, has been a significant advance. These resins offer considerably improved retention of mechanical properties after exposure to moisture, or aqueous acids and bases. For composites based on corrosion resistant resins, exposure to boiling water for two hours is no longer considered an adequate criterion of performance, since the resin itself is capable of much greater resistance to hydrolysis. In order to distinguish performance enhancement imparted by a given coupling agent, a 24-hour residence in boiling water is considered more representative. This is based on the observation that exposure (e.g., 72 hour boiling water) generally does not lead to further strength reduction of a composite containing no coupling agent (i.e., corrosion resistant polyester control).
It has now been found that a silane coupling agent, comprising the reaction product of an isocyanatoalkyl ester with an aminoorganosilane, can impart superior moisture resistance to mineral-filled unsaturated polyesters, as well as other unsaturated resin composites.